The invention relates to a circuit for producing a period parabolic signal comprising a squaring stage for changing a sawtooth-shaped signal having a trace at a retrace and the same frequency.
Such a circuit is known. Its purpose is for example to obtain the parabolic control voltage of field frequency in color television receivers for the east-west modulator with which the distortion in the horizontal direction of the displayed picture is corrected. Another application thereof is producing the parabolic voltage for the dynamic focussing for a picture display tube or for a pick-up tube. An advantage of the method of multiplying two sawtooth-shaped signals relative to the known integration method of a sawtooth-shaped signal by means of a low-pass filter is that a disturbing low-frequency oscillation phenomena cannot occur, whereas, furthermore, the circuit is easily integratable in a semiconductor body. A further known squaring method is the use of a fullwave rectifier which, however, has the drawback, relative to the multiplying method, that it produces more harmonics.
With the integration method the edge occurring during the retrace period is also integrated to a parapolic shape. Relative to the parabola occurring during the trace period, this parabola has the opposite concavity and a much smaller amplitude, as the retrace period is many times shorter than the trace period. The two parabolas obtained have a continuous transition. However, by means of the squaring method a disturbing voltage peak is produced during the retrace period. This is caused by the fact that the parabola then occurring has the same concavity and the same amplitude as the parabola occurring during the trace period. This voltage peak may produce visible ringing phenomena on the display screen due to the fact that the circuit, formed by the line deflection coil, the line trace capacitor etc. is excited, to which circuit the parabolic signal produced by the present circuit is supplied through the modulator. Namely, at the top of the screen vertical straight lines are displayed as undulating lines, and this more at the sides of the screen then in the middle thereof.
It is clear that the disturbing ringing can be damped by means of a resistor disposed in a suitable place in the deflection circuit. However, a considerable amount of energy is lost in this resistor, while the proper operation of the modulator can be seriously disturbed. Another solution is to make the present circuit or the modulator, for example by means of a suppression circuit, inoperative during the field retrace period. A drawback thereof is that steep edges and, consequently, also the ringing phenomena may yet be produced.